In our solar system Jupiter is the king of planets. It is 2.5 times more massive than the other planets combined. But it isn’t the most massive planet we know of. In the search for planets around other stars, we’ve found planets with masses up to 20 times that of Jupiter. All things being equal, we can imagine Jupiter-like planets having a fairly even distribution of sizes, but it turns out that’s not the case. 

A team of astronomers looked at the mass of hundreds of Jupiter-type planets, and found they tend to fall in two groups. One with masses of 1 – 4 Jupiters, and another with much larger masses. Large-mass Jupiters were found around all types of stars, but small-mass Jupiters were only seen around stars with a higher metallicity.

Planet mass vs metallicity for the analyzed stars. In the plot one can see the position of the two populations of giant planets. Credit: Santos et al. 2017

Stars are mostly hydrogen and helium. In astronomy other elements are referred to as “metals.” The metallicity of a star is a measure of how much of these metals a star has. The higher the metallicity, the more metals. Our Sun, for example, has a relatively high metallicity. The team found that metal rich stars like our Sun tend to have planets in the 1 – 4 Jupiter-mass range, while metal poor planets tend to have planets with 4 – 20 Jupiter masses.

The key to this mass difference could be in the way they form. There are two major ideas about how large planets can form. One model is the core accretion model, where a dense metal core forms first, and its gravity then attracts surrounding gas and dust to form a large planet. The other is the gravitational instability model, where gas and dust over a large area becomes gravitationally unstable and collapses under its own weight. It seems that the smaller Jupiters form via core accretion, which limits their mass, while larger Jupiters form by gravitational instability, which allows them to grow quickly.

It’s a bit early to conclude that formation mechanism is the cause of the two mass types. We’ll need to look at a larger sample of planetary systems to be sure. But this work does demonstrate the role of metallicity in planetary formation. Our solar system is just one example of a diverse range of planetary systems, and the formation of these systems was deeply dependent on the characteristics of their home stars.

Paper: N. C. Santos, et al. Observational evidence for two distinct giant planet populations. Astronomy & Astrophysics Vol. 603, A30, DOI: 10.1051/0004-6361/201730761 (2017)

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Io is a violent world. Tortured by the gravitational forces of Jupiter, it erupts with sulphur and lava. Our own planet has volcanoes and lava flows, but it pales in comparison to Io. 

From Earth, astronomers can observe changes in the brightness of Io’s surface, but we had to send probes such as Voyager to Jupiter to see active and erupting volcanoes on Io. This proved that Io is the most geologically active body in the solar system. We can infer much from Io by comparing it to volcanic activity seen on Earth, but there are significant differences. Earth is a water-rich planet with a significant atmosphere, while Io is dry with little atmosphere. This raises questions about how things like lava flows behave on the small moon.

On the surface of Io, Loki Patera is a hot spot that brightens and dims every 400 – 600 days. The most popular explanation is that Loki Patera is a lava lake more than a million times larger than any on Earth. But even high resolution images of Io from the Galileo mission have failed to confirm this idea.

Recently, astronomers used a fortunate celestial event to solve this mystery. Io is the closest Galilean moon to Jupiter. Periodically the next closest Galilean moon, Europa, passes in front of Io as seen from Earth (known as a transit). As it does so, Io’s surface is gradually blocked and revealed by icy Europa. This lets astronomers create a map of Io, particularly Loki Patera. By observing Io during a transit of Europa, they found that the brightness and temperature of the region increased steadily from one end to the other.

Map of Loki Patera showing the variation in brightness and temperature. Credit: K. de Kleer, et al.

This is consistent with the behavior of a lava lake that is overturning. That is, cold lava at the surface of the lake sinks, churning hotter lava to the top. The astronomers also confirmed an island in the center of Loki Patera that has been there since Voyager photographed the region in 1979. This again supports the lava lake model. All it took to verify a lava lake on Io was a fortunate transit by an icy world.

Paper: K. de Kleer, et al. Multi-phase volcanic resurfacing at Loki Patera on Io. Nature 545, 199–202 doi:10.1038/nature22339 (2017)

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A prominent feature of Jupiter’s surface is the Great Red Spot, which has been observed continuously since the 1830s. Jupiters red spot was easy to discover because of its prominent coloring. Now a new spot has been discovered that can only be seen in the infrared. 

Dubbed the “Great Cold Spot,” it exists not in the thick clouds of Jupiter, but in its thin upper atmosphere, a region driven by Jupiter’s strong magnetic field.  Jupiter magnetic field causes ionized particles to stream toward its magnetic poles, creating aurora similar to those we see on Earth. When ionized particles strike Jupiter’s upper atmosphere, it causes that region of the upper atmosphere to heat up. It’s generally been thought that the temperature of the upper atmosphere was rather static, with its temperature cooling as you move farther from the polar regions. But detailed infrared images show Jupiter’s upper atmosphere is complex and dynamic.

The Great Cold Spot (indicated by the white arrow) it seen near the hot region of Jupiter’s magnetic pole. Credit: Tom Stallard/ESO

Using data from NASA’s InfraRed Telescope Facility, a team analyzed the thermal properties of Jupiter’s upper atmosphere near the polar region. They found a cold region similar in size to the Great Red Spot. With data gathered over 15 years, the team could see the cold spot evolve and change, growing and shrinking over time. This indicates that the feature is persistent and dynamic, similar to the Great Red Spot. Thus the upper atmosphere has a complex weather system similar to that of Jupiter’s lower atmosphere.

It remains to be seen whether there are other long-lasting features of Jupiter’s upper atmosphere, but for now we can say that parts of it can be pretty cool.

Paper: Tom S. Stallard, et al. The Great Cold Spot in Jupiter’s upper atmosphere. Geophysical Research Letters. doi: 10.1002/2016GL071956 (2017)

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The Juno spacecraft has captured detailed images of Jupiter’s north pole for the first time. It’s churning, mottled patterns are quite different from the familiar Jovian clouds. 

On the one hand this is somewhat to be expected. Jupiter generates more heat in its interior than it receives from the Sun, and so a pattern of convection forms, where warmer material rises to the surface, then cools and sinks again to the depths. Near the equatorial region Jupiter’s fast rotation smears out these patterns into the familiar band patterns, but near the poles there is no great rotation to create a banded pattern.

On the other hand, the view is quite unexpected. Saturn, likewise, generates more heat than it receives, but on Saturn the banded patterns still exist close to the poles. Then there is the fact that Saturn’s north pole has a hexagon pattern not seen on Jupiter. It’s not clear why the two gas planets should have such different poles.

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The Sun emits a stream of ionized particles known as the solar wind. Generally the solar wind travels at supersonic speeds, meaning they travel faster than sound waves can travel in the wind (though there isn’t much sound in space). But when solar wind collides with the magnetic field of a planet, it slows down significantly. This transition from supersonic to subsonic speeds is known as a bow shock. 

Bow shocks also occur when interstellar wind interacts with a star. Credit: NASA.

Recently the Juno probe crossed Jupiter’s bow shock on its way to the planet. You can hear the transition in the electromagnetic waves measured by Juno. It’s a clear indication that Juno has entered Jupiter’s magnetosphere. Bow shocks are pretty common in astrophysics. Earth’s magnetic field creates one, as do several other planets. Bow shocks can also occur around stars, where the interstellar wind collides with the stellar wind of a star, though interestingly our Sun might not have one.

Juno’s primary mission is to study the electromagnetic and gravitational, so crossing Jupiter’s bow shock is a symbolic milestone, indicating that Juno’s mission is now fully underway.

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On Monday amateur astronomer John Mckeon captured an unusual flash on the limb of Jupiter while he was filming the planet through a telescope. He posted the video on Reddit, wondering if it might be some sort of meteor or comet impact. It was soon confirmed that the flash was also observed by Gerrit Kernbauer. It does indeed appear to have been an impact event. 

Such impact events have been recorded before. Similar impacts were recorded by amateurs in 2010 and 2012. Because such events are unexpected, we rely upon the luck of observers. While such impacts happen fairly regularly, the only time we had a heads-up on an impact was when comet Shoemaker-Levy 9 impacted Jupiter under the watchful eye of Hubble and other telescopes.

Since comet and meteor impacts occur on Jupiter so regularly, it’s a commonly held idea that Jupiter actually protects Earth from comet collisions by deflecting them, or even colliding with them. But in fact Jupiter can deflect a comet toward us just as easily as it can away from us. So the role of Jupiter as Earth’s protector is still unclear, though it’s suspected that it did play a role in the early stages of our solar system.

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On January 7, 1610 Galileo Galilei pointed a small twenty-power telescope at Jupiter. What he observed changed the way we understood the universe.Galileo noticed what appeared to be three small stars near Jupiter. The next evening he again observed three faint stars, but they now appeared on the other side of the planet. Over the next several weeks he watched up to four faint stars weave back and forth near Jupiter. Galileo named them the “Medicean Stars” in honor of his patron Cosimo II de’ Medici, but we now know them as the Galilean moons of Jupiter. 

At first it wasn’t clear to Galileo what these “stars” were, or why they were always found near the king of planets. But Galileo was a patient and accurate observer, and over time it became clear that the motion of these objects followed Kepler’s laws. The same laws that described the motion of the planets around the Sun. Clearly they orbited Jupiter in much the same way as our Moon orbits the Earth. And if moons could orbit a planet, then perhaps it was true that the Earth orbited the Sun after all.

A comparison of Galileo’s observations of the moons with their actual positions as determined from modern measurements. Galileo’s observations are astoundingly accurate. Credit: Ernie Wright

With this discovery and his observations of the phases of Venus later that same year, Galileo gave us proof of a heliocentric universe. Earth was not fixed at the center of the cosmos, but rather moved around the Sun just as other planets did. But Galileo’s discovery not only changed our view of the heavens, it also changed the Earth. Quite literally.

One of the great challenges of cartography has been determining just where on Earth you are. Determining your latitude can be done by observing the position of the stars. For example, the angle of the “north star” Polaris above the horizon is a good basic indication of your latitude. Determining longitude, however, is a very different matter. The Sun, planets and stars travel east to west across the sky, and so there is no clear point of reference for measuring longitude. To make an accurate longitude measurement, you need an accurate clock you can use to measure when particular stars pass overhead, for example. Since the Earth rotates at a steady rate, a time measurement can be used to determine your position east or west of a reference location.

Left: Willem Bleu’s 1650 map of Europe. Right: Robert Janvier’s 1764 map of Europe.

Galileo realized that since the moons of Jupiter obeyed Kepler’s laws, they could serve as a kind of heavenly clock. A clock more precise than any human-made clock of the time. So he began to compile a table of eclipses of the Galilean moons. That is, when a particular moon would pass into Jupiter’s shadow or reappear from behind Jupiter. In 1668, Giovanni Domenico Cassini improved upon these tables, creating a time table accurate enough for cartography. For the first time cartographers could make truly accurate longitude measurements. Many of the accepted distance between cities (used since the Roman Empire) were found to be off by hundreds of miles. The affect of Galileo’s moons can be seen in the difference of world maps made before and after Cassini’s tables.

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Yesterday I wrote about how we know Enceledus has liquid water by its cryovolcanoes. There are other ways we can tell if a moon has a water interior, but one of the more interesting methods is to look at a moon’s aurora. This approach was recently used to show that Jupiter’s moon Ganymede has more liquid water than Earth.

You might have seen an aurora on Earth when there has been a rise in solar activity. On Earth, the aurora (or northern lights) are caused by charged particles emitted by solar flares that interact with Earth’s magnetic field. When a charge enters Earth’s magnetic field, the magnetic field causes the charge to spiral along the magnetic field. For this reason magnetic fields tend to trap charges along their field lines. The charges generally stay trapped unless they collide with each other (not likely in interstellar space) or interact with something else, such as our atmosphere. Where the particles strike our atmosphere depends upon the energy of the charged particles, and thus the activity level of the Sun. As the activity level of the Sun varies, the latitude at which aurora are most prominent can vary.

A Hubble image of aurora superimposed on an image of Ganymede. Credit: NASA/ESA

Aurora have been observed on other planets, as well as moons such as Ganymede. The difference is that the aurora of Ganymede are driven by the interaction with Jupiter’s magnetic field rather than the Sun. The process is much the same as aurora on Earth, but the latitude at which they are observed depends upon fluctuations of Jupiter’s magnetic activity. This has been known for a while, but in this new work the team demonstrated that the latitude variations of Ganymede’s aurora can be used to study the moon’s interior.

Since Ganymede’s aurora are driven by the activity of Jupiter, the team could calculate the amount of variation one would expect given the strength of Ganymede’s magnetic field, giving a variation of about 6 degrees. However, the observed variation is only about two degrees. It would seem that something is dampening latitudinal oscillation of the aurora. One mechanism for this kind of dampening is an interior ocean of saline water. Salt water is a good conductor of electricity, so as Jupiter’s magnetic field varies, it induces a magnetic field in addition to Ganymede’s regular magnetic field. As a result, there are less latitudinal fluctuations in aurora.

The team created a model of this interior ocean. They found that without an ocean there would be a fluctuation of about 6 degrees, but with an interior ocean the fluctuations are lessened. Given an observed fluctuation of about 2 degrees, the interior ocean would need to be about 100 kilometers thick, starting about 150 kilometers beneath the moon’s surface. That means Ganymede has about 70% more water than Earth.

Paper: Saur, J., et al. (2015), The search for a subsurface ocean in Ganymede with Hubble Space Telescope observations of its auroral ovals, J. Geophys. Res. Space Physics, 120, doi:10.1002/2014JA020778.

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In the visible spectrum, Jupiter is a bright, star-like point in the night sky. Viewing it with the naked eye, it would be easy to confuse it with a star except for the fact that it doesn’t twinkle. At radio frequencies Jupiter appears very different. It doesn’t have a simple round shape, for example, and it is extraordinarily bright. So bright that it can outshine the Sun at some radio frequencies.

Much of this radio power is driven by Jupiter’s moon, Io. Tidal forces on Io due to Jupiter’s gravitational field cause the moon to be extraordinarily active geologically. Volcanic activity on Io throws material away from the moon, which tends to spread around Jupiter in a region known as the Io torus. This forms a plasma ring around the planet, through which Io orbits. As Io moves through the plasma torus, it generates a strong electric current between Io and Jupiter. This current is twisted by the rotation of Jupiter, causing a spike in radio intensity about every 10 hours. The radio brightness of Jupiter isn’t a perfect cycle, though, and can vary due to other factors such as solar activity.

Because of the brightness of Jupiter and the ease with which you can create a radio detector, listening to Jupiter is a popular project for student astronomers. For about $100 US, you can build a basic radio telescope to observe radio frequencies between 18 and 28 megahertz, which is pretty optimal for Jupiter. NASA has a project known as Radio Jove, where you can learn about how to build such a telescope, and RadioSky has more details about observing Jupiter, including recorded sounds of the planet.

Usually when we think of amateur astronomy, we think of lens and mirror telescopes for looking at visible objects, but it can also be more akin to ham radio. Just as a first view of Jupiter’s moons can inspire a child’s interest in astronomy, so too can listening in to Radio Jupiter.

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In the first few months of 1610, Galileo Galilei pointed a small twenty-power telescope at Jupiter. What he observed changed the way we understood the universe.

With his telescope, Galileo saw what appeared to be three faint stars in a straight line near Jupiter. The next evening he saw what appeared to be the same three stars, but it seemed Jupiter had moved in the opposite direction to its expected motion. Within a few days it became clear that Galileo wasn’t observing the motion of Jupiter relative to some faint stars, but rather these stars were moving along with Jupiter.

It also became clear that these weren’t stars at all. They didn’t twinkle the way stars do, which was a long known method of distinguishing planets from stars. Stellar twinkling occurs because of small disturbances our atmosphere. Stars are so distant that they appear as points of light, so the atmospheric disturbances bend and deflect the starlight, causing it to twinkle. Planets are close enough that they appear as a small disk. They are too small to resolve as a disk with the naked eye, but they are wide enough that atmospheric disturbances don’t cause them to twinkle.

Galileo knew they must be more like planets than stars, so over the next two months he watched their motion, making a total of 64 observations of their positions. He discovered a total of four objects moving about Jupiter. Sometimes he could observe all of them, and other times he only observed one or two. It was clear, however, that these objects were moving about Jupiter in the same way that the Moon moves about the Earth. Galileo had discovered four moons of Jupiter.

Credit: Ernie Wright

Galileo’s discovery of Jupiter’s moons confirmed that the geocentric view of the universe was wrong. Copernicus had proposed a Sun-centered view of the cosmos nearly 70 years earlier, but this heliocentric model was still controversial when Galileo made his observations. Many supporters of Copernicus were still careful to distinguish the apparent motion of the planets around the Sun from the claim that planets actually moved about the Sun.

Galileo’s moons clearly did not move about the Earth, but instead orbited a mere planet. In the Fall of 1610, Galileo observed that Venus exhibited phases consistent a motion about the Sun rather than the Earth, which further supported the heliocentric model. Of course, claiming that this was evidence that the Earth and other planets actually orbited the Sun famously led him into a bit of trouble with the Church, but that’s a different story.

Perhaps the most amazing aspect of Galileo’s discovery is just how accurate his observations were. Ernie Wright has made a detailed study of Galileo’s moons as they appear in his Sidereus Nuncius (Starry Messenger), comparing them with the actual positions of the moons at the times of Galileo’s observations. You can see two examples of these in the figure above. The agreement is surprisingly good, particularly when you consider that Galileo made his observations with freehand sketches while observing them through a telescope less powerful than a cheap pair of modern binoculars.

The precision of Galileo’s observations more than justifies calling these four moons the Galilean moons of Jupiter.

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In 1665 Gian Domenico Cassini observed a feature on Jupiter.  It was described as an oval about 1/7 the width of the planet itself, and since it appeared reddish in color it became known as the Great Red Spot. Since then the spot has been observed over the years by both professional and amateur astronomers. It has been Jupiter’s most prominent feature for nearly four centuries.

Early on it was thought that the spot was a feature of Jupiter’s surface, and it was used to estimate the rotation of the planet.  We now know that Jupiter is a gas giant, and it rotates differentially like the Sun.  The polar regions rotate a bit more slowly than the equatorial region, and the red spot moves at a slightly different speed than either.  Interestingly, the latitude of the spot has remained fairly consistent over the centuries, at about 20 degrees south of the equator.

The ever shrinking spot. Credit: NASA

We also know that the spot is gradually decreasing in size.  This has been observed since the 1930s, but new results show that seems to be shrinking at an ever faster rate.   It is now shrinking by about 580 km per year. At that rate it could disappear within a couple decades.  Whether it will disappear is still unknown.  We’ve observed storm features like the red spot appear and disappear on Jupiter, but these have been much smaller.

It’s also possible that the spot may disappear for a while only to reappear later.  Although there have been observations of the spot over the centuries, there appears to be a gap from 1713 to 1830 when there doesn’t seem to be any recorded observations of the spot.  That might be because no one was really interested in it, or it could be that the spot wasn’t there.  It is possible that the modern Great Red Spot is not the same as Cassini’s.

If the Great Red Spot does disappear, the general public will likely lament its passing as they lament the demotion of Pluto.  In this case, however, it won’t be the fault of astronomers.

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Jupiter is sometimes said to be a failed star, with a mass just a bit too small to fuse elements in its core.  This isn’t really an accurate description, since the it takes at least 13 times the mass of Jupiter to initiate fusion.  The Sun is a thousand times more massive than Jupiter, so it is unfair to compare Jupiter to a star.  However, when compared to the other planets in the solar system, Jupiter is king.  Jupiter is 2.5 times the mass of all the other planets combined.  With all that mass, it dominates the solar system in more ways than one.

The distribution of asteroids by distance from the Sun has gaps known as Kirkwood gaps.

One of the ways Jupiter dominates is through the asteroid belt.  The gravitational pull of Jupiter gave planetoids within what is now the asteroid belt an extra bit of orbital energy, causing collisions to be more frequent and more violent.  It is part of the reason the belt is a distribution of smaller objects rather than a single planet.  Even today asteroids in the belt are clustered into groups separated by gaps known as Kirkwood gaps.  This gaps occur because of a gravitational resonance with Jupiter.

Another way Jupiter affects our solar system is through its interaction with comets.  Comets can be divided into long period and short period comets.  Long period comets originate from the Oort cloud, and usually enter the inner solar system once, never to be seen again for thousands of years. Short period comets have orbital periods of less than 200 years, and are long period comets that have been deflected by one or more of the outer planets into a shorter orbit.  Usually the planet doing the deflecting is Jupiter.

Impact site of Shoemaker-Levy 9. Credit: NASA

It’s a commonly held idea that Jupiter actually protects Earth from comet collisions by deflecting them, or even colliding with them.  This idea gained popularity when the comet Shoemaker-Levy 9 collided with Jupiter in 1994.  But in fact Jupiter can deflect a comet toward us just as easily as it can away from us.  So the role of Jupiter as Earth’s protector isn’t likely to be accurate.

There is, however, a role Jupiter played that had a significant benefit.  Jupiter happens to have four large moons.  These moons are large enough and bright enough that they can be observed with a small telescope. So in the 1600s when Galileo pointed his telescope at Jupiter, he could observe the four moons orbiting Jupiter.  It demonstrated to Galileo that everything did not orbit the Earth, and pointed him toward a more modern understanding of the Universe.

Up next: Saturn

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